Apparatus for forming silicon carbide filaments

ABSTRACT

Apparatus to form silicon carbide filaments which are essentially uniform throughout their cross-section, which includes a reaction chamber, a thermal screen surrounding the reaction chamber to equalize the temperature therein, a thermal gradient oven concentric with the thermal screen and surrounding the chamber, and a plurality of thermocouples located in the chamber and sensing temperature, and controlling the oven to provide for essentially uniform temperature distribution within the oven, the chamber having inlet means for the admission of organosilane so that conversion of heated tungsten to tungsten carbide is carried out essentially at uniform temperatures throughout the entire chamber; an electrolytic cell follows the chamber in the path of the filament.

United States Patent Combe et a1.

1151 3,658,680 1451 Apr. 25, 1972 [54] APPARATUS FOR FORMING SIL ICONCARBIDE F ILAMENTS [72] inventors: Christian Combe, St. Maur; AndreClouet, Paris; Michel Mnrchal, Palaiseau; Michel Vlllard, Vitry surSeine; Pierre Grenler,

Sceaux, all of France [73] Assignee: Thomson-CSF, Paris, France [22]Filed: Aug. 22, 1969 [21 Appl. No.: 852,410

[30] Foreign Application Priority Data Sept. 4, 1968 France... ..l65067Mar. 26, 1969 France ..698907 [52] U.S. Cl ..204/206, 1 17/106 A,117/106C,

[51] Int. Cl. ...B0lk 3/00, B011: [[00, C231) 3/06 [58] Field of Search..1 18/48-49.5'; l17/107.1, 106 C;204/130, 140, 206

[56] References Cited UNITED STATES PATENTS 1,077,696 11/1913 Fuller..'..'.'....2o4/14o 'IIIIIIIII- 1,731,269 10/ 1 929 Rich .;204/14O3,409,469 11/1968 Kuntz ....l17/107.l

3,424,603 H1969 Schwartz ..1 17/107. 1

FOREIGN PATENTS OR APPLICATIONS 402,893 12/ l 933 Great Britain ..204/ l40 Primary Examiner-John H. Mack Assistant Examiner-T. TufarielloAttorney-Flynn and Frishauf 1 ABSTRACT Apparatus to form silicon carbidefilaments which are essentially uniform throughout their cross-section,which includes a reaction chamber, a thermal screen surrounding thereaction chamber to equalize the temperature therein, a thermal gradientoven concentric with the thermal screen and surrounding the chamber, anda plurality of thermocouples located in the chamber and sensingtemperature, and controlling the oven to provide for essentially uniformtemperature distribution within the oven, the chamber having inlet meansfor the admission of organosilane so that conversion of heated tungstento tungsten carbide is carried out essentially at uniform temperaturesthroughout the entire chamber; an electrolytic cell follows the chamberin the path of the filament.

2 Claims, 1 Drawing Figure rl/pliril) APPARATUS FOR FORMING SILICONCARBIDE FILAMENTS The present invention concerns the production ofcontinuous filaments of silicon carbide having extremely high mechanicalresistance evolved from a core of tungsten, graphite or any othermaterial that is conductive or that may be rendered surface-conductive.The invention is also concerned with a method for forming suchfilaments.

BACKGROUND OF THE INVENTION Silicon carbide filaments can be used asreinforcement elements, whether in the form of a fiber sunk in a metalor in a resin, or as a threadlike support for a variety of bases, so asto furnish composite materials having remarkable mechanical or surfaceproperties.

Known techniques for obtaining continuous filaments of silicon-carbideaim at inducing the deposit of this carbide by decomposition of chemicalvapors on contact with an electrically conductive wire heated to a hightemperature.

Silicon carbide has been obtained by pyrolytic decomposition on contactwith a hot tungsten wire of organo-silane vapors transported by hydrogenor mixed hydrogen-diluent gas current. In the course of this process,the diameter of the tungsten wire increases by reason of the formationof the deposit, and it is necessary to progressively increase power toheat the tungsten wire in such a way as to bring back the surfacetemperature of the filament to a value for which the speed of increaseof the deposit is substantial.

Filaments obtained hitherto by this method have in crosssection amicrostructure consisting of concentric Stratifications. Thesestratifications correspond to differences of microstructure depending ondeposit conditions and in particular to temperature; each time that itis necessary to adjust the heating power in order to renew anappreciable speed in the increase of the deposit, the formation of a newlayer is induced, whose structure is different from that of theimmediately preceding layer.

This method leads to the appearance of strains between the differentstrata which are susceptible of causing cracks in the deposit.

The present invention has as its principal object improvements in thismethod with a view to manufacturing filaments whose structure isregular. Another object of the present invention is to provide a devicefor use with this method.

SUMMARY OF THE INVENTION In accordance with the invention, there isprovided a method for making a silicon carbide filament by pyrolyticreaction of a gaseous organosilane in contact with a heated tungstenwire within a reaction zone as the wire is moved through the zone. Thetemperature of the wire along its length within the zone as it is movedthrough the zone is substantially constant, and the temperature of thezone is also maintained substantially constant. It is also advantageousto keep substantially constant the gaseous composition, the dischargetemperature, and the velocity of the wire through the zone.

In accordance with the invention, there is also provided a reactor foruse with said method. The reactor is characterized by a reaction chambersurrounded in part at least with a thermal screen and a thermal gradientoven concentric with each other and an electrolytic cell secured to andcommunicating with the discharge end of said chamber.

BRIEF DESCRIPTION OF THE DRAWING Advantages of the invention will becomeapparent to those skilled in the art from the following descriptionconsidered in conjunction with the drawing which is a schematic flowdiagram illustrating the preparation of a silicon carbide filament.

DESCRlPTION OF THE SPECIFIC EMBODIMENTS it is preferable that thegaseous mixture should flow parallel to the wire in the reactionchamber.

The wires surface temperature can be between about l,100 C. and about1,400 C; preferably, variations do not exceed 30 C. along the length ofthe wire in the chamber. The most favorable temperature in the case of agaseous mixture charged with evaporated methyltrichlorosilane cn sicn isbetween about 1,200" C. and about 1,300 C. In order to maintain thesurface temperature of the wire at its proper value after thermalequilibrium has been established in the reaction chamber, it isparticularly advantageous to stabilize the overall temperature of thechamber and that of the wire by controlling the wires heating current bymeans of temperature sensors or transducers (e.g., thermocouples)located at different selected points in the reaction chamber.

In accordance with an important characteristic of the present invention,a thermal screen, which can be made of a reflecting metal (e.g.,aluminum), is placed around the reaction chamber and along an adjustablelength measured from the extremity of the chamber from which the wireissues. The screen has the effect of reducing inequalities oftemperature along the wire which, in the absence of the screen, would becolder at the exit end of the chamber because the silicon carbidedeposit would necessarily be thicker at this end.

The gaseous mixture can be evacuated at atmospheric pressure and beintroduced under conditions closely approximating normal conditions oftemperature and pressure. Nevertheless, it is advantageous to providefor thermal regulation of the chamber in which the gaseous mixture isformed.

At the exit from the deposit chamber in which the coating of siliconcarbide is achieved, the wire enters a vessel in which the electrolytictreatment is effected. Although this arrangement is preferred, it isalso possible to collect the wire on a receptor spool and subsequentlysubject it to electrolytic treatment.

The method in accordance with the present invention can be executed withthe apparatus represented schematically in the drawing. This apparatusincludes, with regard to deposit of silicon carbide, two cylindricalchambers l and 2 preferably of glass disposed on a more or lesshorizontal axis and joined to each other by the horizontal branch of amercury trap 1:; the nonadjacent extremities of chambers 1 and 2 issueinto the horizontal branches of two other mercury traps J l and J;,. Themercury traps utilized here reflect a special concept, so that thetemperature of the mercury can be maintained at a low value, of theorder of from about 40 C. to about 60 C. and the well-known lgnitroneffect can be totally eliminated here.

The extremities of the horizontal branch of the mercurycontaining glasstrap J are joined to capillary tubes traversed by the wire. Capillaryforces suffice to prevent any escape of mercury by way of the annularspace existing between the wire and the inner wall of the capillarytube. As illustrated by the drawing, a tungsten wire issues from areeling drum 3 and unwinds through the chambers by way of the mercurytraps. As will be shown in greater detail below, the wire is transformedinto a silicon carbide filament in the course of its transit withindeposit chamber 2, and, issuing from chamber 2 in this new state, itenters chamber 9 where it undergoes electrolytic treatment.Alternatively, the silicon carbide filament can also be stored on areceptor spool (not shown) for subsequent introduction into structure 9.

The wire in chambers 1 and 2 is heated by Joule effect by means ofappropriate sources of electric current (not shown), the source beingconnected to the wire in chamber 1 by the mercury'of traps J and J andto the wire in chamber 2 by the mercury of traps J and J Any type ofelectrical contact sliding on the wire can also be used to the sameeffect, but the mercury traps are preferable.

A brake (not shown) can be placed to advantage on discharge spool 3. Atensioning device can be used to stretch the wire, at high temperature,so as to eliminate any torsion of the wire which could result from theway in which the wire is drawn.

The first chamber 1 is a chamber for cleaning the wire in an atmosphereof hydrogen. It is equipped with inlet line E connected with a hydrogensource (not shown) and exit line 5 communicating with the atmosphere.The role of the first chamber 1 is to clean the surface of the tungstenwire of all grease or any organic material and to free this surface ofany occluded oxygen. In chamber 1, the wire is heated to a temperatureof between about 800 C. and about l,200 C. The cleaning operation isnoteworthy in that the wire passes directly into the treatment chamber 2without returning to the outside air.

A gaseous mixture is charged from vessel 5, which contains a liquidorgano-silane, preferably methyltrichloro-silane. The

mixture is made up of silane vapors drawn by bubbling a hydrogen currentfree of water vapor under normal temperature and pressure conditionsinto vessel 5. The composition of the mixture 'is carefully controlledand kept constant by insulation of vessel 5 and thermal regulation in athermostat controlled bath 6. The gaseous mixture is passed throughfilter 7 before entering depositchamber 2.

In accordance with the invention, a substantially uniform deposit ofsilicon carbide is formed by pyrolytic reaction of the silane-containinggaseous mixture on contact with the hot wire, the surface temperature ofthis wire being maintained practically constant throughout the length ofthe wire inside deposit chamber 2 during the entire duration of theprocess. The temperature, which depends on the silane utilized, isbetween about l,000 C. and about l,400 C. with a minimum variation of 30C. between any two points of the wire. The most favorable temperatureused with methyltrichlorosilane is between about l,200 C. and about1,300 C. At the same time, the temperature prevailing in deposit chamber2 is likewise kept substantially constant,.as are the dynamic flowcharacteristics of the gaseous mixture with regard to any point of thewire within chamber 2; it has already been mentioned above that thecomposition and the temperature of the gaseous mixture are controlledbefore their entrance into chamber 2.

By reason of the operating conditions, gas intake and exit lines E and Srespectively, of chamber 2 are oriented so as to direct the gases alongthe length of the wire. This prevents local cooling in the wire andfavors a stable temperature of the gaseous mixture as well as itslaminar displacement around the filament.

As a modification, notably in function of the heating power of the wire,the gases can be introduced in the same direction as the movement of thewire, as is indicated in the drawing by arrows, or in the oppositedirection. Gas flow is best con-' ducted under normal atmosphericpressure. This is why deposit chamber 2 is surrounded, to an adjustablelength counted from the wire exit end thereof, by thermal screen 8 whichcan, for example, be made of a reflecting metal, and by an oven having athermal gradient. ln the area of its extension, screen 8 and oven 15compensate for heat losses occuring by way of the walls of depositchamber 2. By regulation of the temperature gradient and position ofoven 15 and by observing the surface temperature of the wire, forexample by means of a pyrometer (not shown), temperature variationsalong the wire can be limited so as to maintain this temperature withinthe limits of plus or minus 15 C. Along the entire length of the wire inchamer 2.

When, within deposit chamber 2, overall thermal equilibrium isestablished, for which the wire possesses the selected temperature, thisequilibrium is controlled by temperature points at various points of thechamber by means of thermocouples, of which one (Th) is represented inthe drawing.

A single deposit chamber 2 has been represented, by way of simplifyingthe figure, but it is preferred to arrange several similar depositchambers in series. The reactive gaseous mixture can be distributed ineach chamber individually, but it is also possible to make the gasespass directly from one chamber to the other. The number of depositchambers and the length of each of them are directly related, thecombination of the two determining the final diameter of the siliconcarbide filaments produced.

By way of illustration, a more detailed example is now given of themethod whereby the methyltrichlorosilane is decomposed to produce asilicon carbide filament.

A tungsten wire having a diameter of from about 10 to about 20 micronsand moving at a linear velocity of from about 0.2 to about 2 meters perminute, is introduced into chambers 1 and 2. Cleaning chamber 1 ischarged with pure hydrogen containing less than about 30 parts permillion (ppm) of water vapor via line E Deposit chamber or chambers 2are charged with a reactive gaseous mixture containing about 15 volumesof vaporized methyltrichlorosilane per volumes of hydrogen via line EThe reactive gases are passed in the direction of the wires movement ata velocity of approximately 1 meter per minute; these gases escapethereafter into the outside air through line 8;.

Wire velocityas well as composition and discharge of the gaseous mixtureare kept constant.

A heating current is introduced into the wire between mercury traps J,and J on the one hand, and J and 1;, on the other. The temperature ofthe wire in cleaning chamber 1 is maintained at approximately l,250 C.

The temperature of the wire in deposit chamber 2 is maintained atapproximately l,250 C. As soon as indications given by severalthermocouples Th, of which only one is shown, placed at representativepoints within the deposit chamber 2 show that thermal equilibrium hasbeen achieved within this chamber,.oven 15 is adjusted so thattemperature variations along the wire will not exceed about 20 C. toabout 30 C. The heating current is then regulated so that the averagetemperature of the wire can be as close as possible to 1,240" C. depositchamber 2 being in thermal equilibrium.

Thereafter, the thermocouples placed in deposit chamber 2 permit theregulation of the heating current so that the thermal equilibriumtemperature of chamber 2 will remain practically unchanged during theentire process.

The final diameter of the silicon carbide filaments depends on thelength of time the wire stays in the deposit chamber or chambers 2. Itis usually agreed that a final diameter of the order of from about 100to about 200 microns is a satisfactory compromise between the volume ofthe tungsten, the overall volume of the wire, the mechanical behavior ofthe silicon carbide filament and its utilization as a reinforcementelement.

The linear speed at which the wire moves through the system can varywidely, as from a few centimeters to tens of meters per minute.Adjustment ranges and conditions for the temperature and 'gas dischargein the chambers are increasingly narrow in proportion to the increase invelocity with which the filament moves through the system.

The layer of silicon carbide is deposited in concentric fashion,resulting in a fairly circular cross-section in the product obtained.The tungsten filament is embedded in the central portion and is moreoften than not decomposed into fragile carburization products. Thepolycrystalline state of the silicon carbide deposit in forms alpha andbeta-and beta-is highly pronounced, since the diameter of elementarycrystallite attains values of the order of 0.025 micron. The mechanicalresistance of the filaments obtained depends essentially on theconditions for depositing the silicon carbide and the uniformity of thesurface state. At this stage of the process, the breaking strengthattains commonly average values of from about 220 to about 240 kg./mm.values due to the excellence of the surface state. However, despite thisexcellent surface state whose irregularities have a depth generally notexceeding 0.02 micron, it has been found to be advantageous to provide asuper-fine finish to the filaments surface by electrolytic treatment. Asuitable technique comprises mounting in series after Chambers 1 and 2,electrolysis cell 9 made up of a metallic tube 10, preferably ofstainless steel, provided with two plugs 11 and 12 each of which istraversed by a capillary tube through which the wire is passed.

An electrolyte is charged to tube 10 through line 13; the electrolyteleaves tube 10 by line 14 nd is recycled to the tube by a pump (notshown). Tube 10 is carried, with regard to the wire, to a negativepotential by means of a feed mechanism (not shown). At the exit of cell9, the wire is rolled onto a receptor spool 4 actuated by a motor (notshown). For a tube of stainless steel whose diameter is approximatelymm. and length 400 mm., advantageous results have been obtained underthe following conditions:

Electrolyte: 5% KOl-l solution Temperature: 20 C. to C.

Duration of contact between wire and elecrolyte: 2 minutes Difference ofpotential between wire and tube: 12 volts.

Before subjecting the wire to this electrolytic treatment, breakingstrengths ranging in average value between about 220 and about 240kgJmm. are commonly observed, the breaking test being effected onsamples whose length is of the order of 100 mm.

Following the wires electrolytic treatment, breaking strength commonlyincreases to from about 20 to about percent, and average values rangingbetween about 260 and about 320 kg./mm. are attained, the modulus ofelasticity remaining equal on the average of 45,000 kg./mm

These mechanical properties explain the excellent rigidity andelasticity of the silicon carbide filament as well as ability to acceptbending with very small curvature.

While methyltrichlorosilane is preferred organosilane, related silanescan also be employed such as: dimethydichlorosilane andtrimethylchlorosilane.

The preceding description is given as a nonlimiting example and thepresent invention encompasses such variations or modifications asdefined by the language of the appended claims.

What is claimed is:

1. Apparatus for the treatment of a tungsten filament to form a siliconcarbide filament comprising an elongated reaction chamber (2) havinginlet and outlet means for passing a filament of tungsten therethrough,and removing the treated filament from said chamber;

a thermal screen (8) surrounding the chamber to equalize the temperaturewithin the chamber;

a thermal gradient oven (15) concentric with the thermal screensurrounding said chamber;

a plurality of thermocouples (Th) located within the chamber and sensingthe temperature at selected locations therein, the thermocouples beingconnected to the thermal gradient oven to regulate the heat gradient ofthe oven and thus the average temperature within the oven;

means (1,) connected to the outlet means of the chamber to isolate theatmosphere within the chamber;

means introducing and removing gases to, and from the chamber;

and an electrolytic cell (9) located to follow, in the path of thefilament, said isolating means.

2. Apparatus according to claim 1, including means (5, 6) supplying agaseous silane mixture to the chamber, said means comprising aninsulated vessel (5) and a thermostatically controlled bath (6)surrounding said insulated vessel and maintaining the temperature ofsaid vessel constant.

2. Apparatus according to claim 1, including means (5, 6) supplying a gaseous silane mixture to the chamber, said means comprising an insulated vessel (5) and a thermostatically controlled bath (6) surrounding said insulated vessel and maintaining the temperature of said vessel constant. 